Method of cleaning residual liquid developer from electrophotographic plates

ABSTRACT

IN LIQUID DEVELOPMENT SYSTEMS, CYCLING ELECTROSTATOGRAPHIC IMAGING SURFACES ARE SUBSTANTIALLY COMPLETELY CLEANED OF DEVELOPER REMAINING ON THE SURFACE AFTER TRANSFER TO COPY PAPER BY CYCLICALLY CONTACTING THE IMAGING SURFACE WITH A SMALL QUANTITY OF HIGHLY ABSORBENT DRY POWDER AND REMOVING THE POWDER FROM THE IMAGING SURFACE AFTER THE POWDER HAS ABSORBED THE RESIDUAL LIQUID DEVELOPER.

U nitcd States Patent 3,697,263 METHOD OF CLEANING RESIDUAL LIQUID DEVELOPER FROM ELECTROPHOTOGRAPHIC PLATES Joseph Mammino, Penfield, N.Y., assignor to Xerox Corporation, Rochester, N.Y. No Drawing. Filed Oct. 31, 1969, Ser. No. 873,103 Int. Cl. G03g 13/22 U.S. Cl. 961.4 11 Claims ABSTRACT OF THE DISCLOSURE In liquid development systems, cycling electrostatographic imaging surfaces are, substantially completely cleaned of developer remaining on the surface after transfer to copy paper by cyclically contacting the imaging surface with a small quantity of highly absorbent dry powder and removing the powder from the imaging surface after the powder has absorbed the residual liquid developer.

BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more particularly, to improved cleaning systems and techniques.

The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finelydivided electroscopic material referred to in the art as toner. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to a support surface as by heat. Instead of latent image formationby uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image directly by charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

Similar methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. Included within this group are the cascade development technique disclosed by E. N. Wise in U.S. Pat. 2,618,552; the powder cloud technique disclosed by C. F. Carlson in U.S. Pat. 2,221,776 and the magnetic brush process disclosed, for example, in U.S. Pat. 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern, the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration.

A further technique for developing electrostatic latent images is the liquid development process disclosed by R. W. Gundlach in U.S. Pat. 3,084,043 hereinafter referred to as polar liquid development. In this method, an electrostatic latent image is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions or lands defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions or valleys. The depressed portions of the developer dispensing member contains a layer of conductive liquid developer which is maintained out of contact with the electrostatographic imaging surface. Development is achieved by moving the developer dispensing member loaded with liquid developer in the depressed portions into developing configuration with the imaging surface. The liquid developer is believed to be attracted from the depressed portions of the applicator surface in the charged field or image areas only. The developer liquid may be pigmented or dyed. The development system disclosed in U.S. Pat. 3,084,043 differs from electrophoretic development systems where substantial contact between the liquid developer and both the charged and uncharged area of an electrostatic latent image hearing surface occurs. Unlike electrophoretic development systems, substantial contact between the polar liquid and the areas of the electrostatic latent image bearing surface not to be developed is prevented in the polar liquid development technique. Reduced contact between a liquid developer and the non-image areas of the surface to be developed is desirable because the formation of background deposits is thereby inhibited. Another characteristic which distinguishes the polar liquid development technique from electrophoretic development is the fact that the liquid phase of a polar developer actually takes part in the development of a surface. The liquid phase in electrophoretic developers functions only as a carrier medium for developer particles. In general the developer technique disclosed in U.S. Pat. 3,084,043 may provide development with liquid developers having a conductivity of from about 10- (ohm-cm)" to about 10- (ohmcm.)-

An additional liquid development technique is that referred to as wetting development or selective wetting as described in U.S. Pat. 3,285,741. In this technique, an aqueous developer uniformly contacts the entire imaging surface and due to the selected wetting and electrical properties of the developer substantially only the charged areas of the imaging surface are wetted by the developer. The developer should be relatively conductive having a resistivity generally from about 10 to 10 ohm-cm. and having wetting properties such that the wetting angle measured when placed on the imaging surface is smaller than at the charged area and greater than 90 at the uncharged areas.

While capable of producing satisfactory images, these liquid development systems in general, suffer deficiencies in certain areas and are in need of further development and improvement. Particularly troublesome difficulties are encountered in liquid development systems employing reusable or cycling electrostatographic imaging surfaces. In these systems, for example, a photoconductor such as a selenium or selenium alloy drum as the photoconductor surface is charged, exposed to a light and shadow image and developed by bringing the image bearing surface into developing configuration with an applicator containing developing quantities of liquid developer thereon. The liquid developer is transferred according to the appropriate technique from the developer applicator onto the image bearing surface in image configuration. Thereafter, the developer pattern on the electrostatographic imaging surface is transferred to copy paper and the liquid developer may be absorbed by the paper to form a permanent print. During the transfer operation not all the liquid developer is transferred to the copy paper and a considerable quantity remains on the photoconductor surface. In order to recycle the imaging surface, this residual developer must be either removed or its effects immobilized otherwise it will tend to be present as background in subsequent cycles. If the liquid developer is relatively conductive having, for instance, a resistivity less than about ohm centimeters, any residue remaining on the imaging surface may dissipate any charge subsequently put on it. Furthermore, lateral conductivity of the liquid developer on the imaging surface may become excessive and the resolution of the resulting image will be poor. On repeated cycling there is also a progressive accumulation of liquid developer on the imaging surface since in each cycle not all the developer is transferred to the copy paper. This progressive accumulation of developer residue results in an overall loss of density, deterioration of fine detail and contributes to increased background deposits on the final copy particularly since accurate imaging on the imaging surface may be inhibited.

Various procedures to remove the developer liquid from the surface of a photoconductor have been proposed, but never with commercial success. However, to provide the necessary removal of ink film, the cleaning step must be so severe and complete that there is a progressive degradation of the photoconductor surface lessening its useful life span. The severity of the cleaning step is dictated by the fact that in cleaning a film from a surface the film is progressively split so that on each separate cleaning about one half the film remains on the photoconductor surface. The cleaning solvents which have been proposed to provide adequate cleaning frequently are major contributors to the chemical attack of the imaging surface and are frequently hazardous due to their volatility and toxicity. In most instances when complete removal of the ink film is achieved by solvent action, the electrical properties of the photoconductor are virtually destroyed by the cleaning operation after only a small number of cycles. In other instances, the celaning solvents employed may act as solvents for the resin binder in a binder plate or may induce crystallization of the thin layer of selenium.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a developing system which overcomes the above noted deficiencies.

It is another object of this invention to provide a novel cleaning system.

. It is another object of this invention to provide a thorough, fast and complete cleaning technique.

It is another object of this invention to provide a cleaning system which does not degrade imaging surfaces.

It is another object of this invention to provide a cleaning system which removes substantially all residual developer from an imaging surface.

It is another object of this invention to provide a simple-cleaning system capable of cyclical use.

It is another object of this invention to provide a dry method of cleaning reusable photoconductors.

It is another object of this invention to provide a liquid development system wherein the photoconductor may be repeatedly recycled without substantial loss of electrical properties.

It is another object of this invention to provide a cleaning system superior to known systems.

It is another object of this invention to provide a liquid development system superior to known systems.

The above objects and others are accomplished, generally speaking, by providing a cycling electrostatographic imaging system having a cleaning system which enables complete cyclic cleaning of residual liquid developer without imaging surface degradation. More specifically, in a liquid development system employing a cycling or reusable electrostatographic imaging surface the residual developer is cyclically substantially completely cleaned from the imaging surface by treating the surface with a small quantity of highly absorbent dry powder.

Any suitable absorbent powder may be employed in the practice of this technique. The cleaning aids of this invention may generally be described as small sized highly absorbent dry powders that functionas tiny sponges. To provide long life for the imaging surface it is preferred that the powder be relatively non-abrasive with respect to the imaging surface. To minimize abrasion, the dry powders preferably are softer than the surface which they are to clean. For maximum cleaning effectiveness, the powders are also preferably not soluble in the liquid developer, are abhesive after they have absorbed the developer liquid thereby enabling ready removal of the cleaning particles from the surface and do not introduce anything to contaminate or degrade the photoconductor.

The absorbent powders typically have a surface area of from about 30 to about 950 m. gm. Particularly effective cyclic cleaning may be obtained with powders having a surface area of from about 100 to about 800 m. gm. These powders are quickly and easily dispersed in residual liquid developer and only a minimum of powder is required to efficiently absorb the developer liquid.

The cleaning powders of this invention may be of any suitable size which provides the necessary surface area and sorptive ability. Typically, the particles range in size from about 0.1 to about 30 microns. As the size of the particles and their hardness increases, they become more abrasive. On the other hand, as the size decreases greater caution must be exercised in the handling and containment of the powders. For these reasons average particle sizes of from about 1 to about 20 microns are preferred with average particle sizes of from about 3 to about 15 microns providing optimum balance in handling and performance.

The particular cleaning powders should, in general, be selected based on the absorptive capacity of the powder for the particular developer. If the developers are oil based material, the cleaning powder preferably has a high oil absorption. If the developer is a polar liquid, the cleaning powder preferably has high absorptive power for the polar developer and may have low oil absorption. Typically, the absorption capacity is from about 40' to about 500 mg. developer/100 mg. powder. Preferably the absorption capacity is from about to about 350 mg. developer per mg. powder to provide adequate cleaning while minimizing powder packing and caking.

Typical absorbent microporous materials useful in the practice of this invention include the finely divided forms of carbon, such as furnace black, channel black, lamp black, bone black, graphite and charcoal; clays such as kaolin and China clay; diatomaceous earth, pumice, mica, fly ash, infusorial earth; pigments such as titanium dioxide, precipitated calcium carbonate, lithopone, iron oxide, silica, zinc oxide, calcium silicate and magnesium aluminum silicate. Other pigments having adequate sorptive capacity include qui-nacridones, phthalocyanines, benzidine yellow, and Hausa yellow. Additional materials functioning as micro-sponges include micro-reticulated plastics such as polyurethane and polyethylene.

Especially satisfactory cleaning is obtained with silica gels, kaolin clays, carbon blacks and titanium dioxide. Particularly preferred materials in handling and cleaning ability are the silica gels having a particle size of from about 3 microns to about 11 microns, a surface area of from about 200 m. /gm. to about 350 mF/gm. and an oil absorption of from about 100 to about 315 mg. per 100 mg. powder.

The absorbent cleaning powder may be applied to the surface to be cleaned in any suitable amount. Typically, the absorbing power of the powder is at least sufiicient to absorb all the residual liquid on the surface. Preferably,

the amount of cleaning powder applied provides an absorbing power greater than that necessary to absorb all the residual liquid. Generally, the residual developer is present on the imaging surface in an amount up to a maximum of about 2.3 mg./cm. About 80% by weight of the developer consists of various liquid components and for such amounts of liquid in the developer from about 1 to about by weight of the developer of the cleaning powders, are generally added.

The cleaning powders of this invention may be applied to the surface to be cleaned in any suitable manner. Typi cally, the powders may be dusted, sprinkled, brushed, cascaded across the surface and applied as a powder cloud. The powders may also be applied by wiping with powder impregnated or coated sheets, webs, papers and cotton wadding. They may also be applied as an aerosol spray. In providing uniform cleaning and to minimize problems of dust, it is preferred to apply the absorbent microporous powders in the form of coated or impregnated webs, sheets or wadding. To minimize wear and abrasion of the surface to be cleaned, the powder is preferably applied with sufiicient mixing on the surface to be cleaned to absorb all the liquid and wiped 01f with sufficient pressure to provide a clean surface.

After mixing the cleaning powders contain substantially all the residual liquid developer and are present on the imaging surface substantially in the form of a liquid loaded powdery residue. This powdery residue of cleaning powder containing absorbed liquid may be removed from the imaging surface in any suitable manner. The powdery residue may, for example, be removed by wiping with a cloth, brush Web or by contact with a wiper blade since the substantially dry powdery residue does not readily adhere to the imaging surface.

Since the cleaning powders and techniques of this invention provide a substantially complete cleaning of residual developer from the imaging surface on every cycle, no problem of charging and imaging through a residual liquid film exists and developers of both low and high conductivity may be employed. Any suitable developer may be used, Typically, the developers for which the cleaning aids of this invention are effective have a conductivity of from about 10- (ohm-cm.)* to about 10* (ohm-cm.)- Typical vehicles within this group providing these properties include water, methanol, ethanol, propanol, glycerol, ethylene glycol, polypropylene glycol, 2,5-hexanediol, mineral oil, the vegetable oils including castor oil, peanut oil, coconut oil, sunflower seed oil, corn oil, rape seed oil and sesame oil. Also included are silicone oil, mineral oil, mineral spirits; fluorinated hydrocarbons such as Du Ponts 'Freon solvents and Krytox oils; silicone oils, esters such as fatty or acid esters, kerosene, and oleic acid. In addition, as is well known in the art, the developers may contain one or more secondary vehicles, dispersants, pigments or dyes, viscosity controlling additives or additives which contribute to fixing the pigment on the copy paper.

Any suitable electrostatographic imaging surface may be cleaned with the technique of this invention. Basically, any surface upon which an electrostatic charge pattern may be cyclically formed or developed may be employed. Typical electrostatographic imaging surfaces include dielectrics such as plastic coated papers, xeroprinting masters and photoconductors and overcoated photoconductors. Typical photoconductors that may be employed include selenium and selenium alloys, cadmium sulfide, cadmium sulfo selenide, phthalocyanine binder coatings and polyvinyl carbazole sensitized with 2,4,7 trinitrofiuorenone. Typical overcoated photoconductors include those described in 'U.S. Pats. 3,251,686 and 3,234,019 and may comprise a selenium layer on a conductive aluminum substrate overcoated with a thin film of insulating material such as polyethylene terephthalate. The electrostatographic imaging surface may be employed in any suitable structure including plates, belts or drums and may be employed in the form of a binder layer. For more elfective cleaning, it is preferred to provide a surface to be cleaned which has a very smooth surface since generally the smooth and more uniform the surface the better will be the cleaning.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following preferred examples further define, describe and compare preferred materials, methods and techniques of the present invention. In the examples all parts and percentages are by weight unless otherwise specified.

Example I A Type E selenium xerographic plate commercially available from Xerox Corporation, Rochester, N.Y., 11 by 16 inches and comprising a surface layer of selenium about 50 microns thick on a conductive aluminum plate is positively charged and exposed to a light and shadow image in conventional manner. The electrostatic latent image thus formed is developed by moving a patterned surface applicator roll having developing quantities of developer in the depressed portions thereof past the image bearing surface so that liquid developer is pulled out of the depressed portions to the image bearing surface in image configuration. The speed of development is about 10 inches per second. The developer employed is of the following composition:

Parts by weight Drakeol 9 30 Microlith CT 18 Methyl violet tannate 3 Ganex V216 l5 Parafiint RG wax 0.5

Drakeol 9 is a mineral oil manufactured by Pennsylvania Refining Company having a kinematic 'viscosity of about 15.718.1 centistokes at 25 C. and a specific gravity of about 0.85. Microlith CT is a resinated predispersed carbon black pigment composed of about 40% carbon black pigment and 60% ester gum resin, manufactured by CIBA. Parafiint RG is a hard synthetic wax available in flake form from Moore & Munger. Ganex V216 is an alkylated polyvinyl pyrrolidone compound, manufactured by GAP Corporation, which serves as additional pigment dispersant and may also be regarded as a secondary vehicle.

The developer is prepared by combining the mineral oil and Ganex V216 in a suitable vessel while stirring, heating to about 100 C. and then adding the pigment and other ingredients while continuing the stirring.

The developer on the photoconductor is transferred to Xerox 4024 copy paper in image configuration. About mg. of Syloid 308, a silica gel available from Davison Chemical Division W. R. Grace & Company is sprinkled from a salt shaker onto the selenium plate. The plate is then gently wiped with absorbent cotton to remove all the residual developer. The plate is observed to be clean, free of all removable developer and free of interference patterns. The first print is free of background and has a resolution of about 10 line pairs per millimeter. The clean photoconductor plate is again subjected to the same cycle of charging, exposing, developing, transferring and cleaning. In the course of making sequential prints according to this cycle no significant change in print quality is observed.

EXAMPLE II Example I is repeated except that after each transfer the silica gel is not added. Instead the plate is wiped with absorbent cotton with equal or greater effort than in Example I. Residual developer in the form of interference patterns in streaks is observed to remain on the drum after each cycle. The first print is free of background and has a resolution of about 10 line pairs per millimeter. After 10 cycles the resolution is observed to gradually decrease to about 3 line pairs per millimeter and by the fifteenth cycle to be about 1 line pair per millimeter.

EXAMPLE III An overcoated photoconductor about 9 inches by 14 inches in dimension comprising a one quarter mill film of polyethylene terephthalate (obtained from E. I. du Pont de Nemours & Company under the trade name Mylar) overcoated on a 20 micron thick layer of selenium on an aluminum substrate prepared according to the procedure of Example I in US. Pat. No. 3,251,686 is charged and exposed to a light and shadow image in conventional manner. Development of the electrostatic latent image is obtained with the developer and in the manner described in Example I. The developer on the overcoated photoconductor is transferred to bond paper in image configuration. The overcoated photoconductor has about 60 mg. of Syloid 30 8, silica gel sprinkled on to its surface. The surface overcoating is gently wiped with absorbing cotton and all the residual developer is removed. The first print is free of background and has a resolution of about 10 line pairs per millimeter. Upon repeated cycling through charging, exposing, developing, transferring and cleaning stations after 50 prints no significance in print quality is observed.

EXAMPLE IV A xeroprinting master 9 x 14 is prepared by placing a thin insulating coating of epoxy resin about .0005 inch thick in image configuration on a conductive plate of aluminum by means of a silk screen stencil, and then hardening the resin in known manner. The plate is charged to +450 volts by passing it under a corona charging unit. The image is developed in the manner described in Example I with a liquid developer having the following composition:

Parts by weight Polypropylene glycol 47 Staybelite Ester MicrolithCT 29 Rucofiex TG-8 19 Staybelite Ester is an esterified wool rosin available from Hercules Powder Co. Rucoflex TG-8 is triethylene dicaprylate available from Hooker Chemical Co.

The developer is transferred to bond paper and the resulting print has image density of about 0.9, background density of less than 0.01 and a resolution of about 7 line pairs per millimeter. The plate is cleaned with about 100 milligrams of Mogul A carbon black pigment available from Cabot Corporation with the technique described in Example I. Thereafter, the xeroprinting master is repeatedly charged, developed With developer being transferred to bond paper and cleaned as described above for 25 cycles. The speed of development is about 12 inches per second and the 25th print obtained is of substantially the same quality as the first print.

EXAMPLE V A xerographic plate comprising a phthalocyanine binder layer in which the binder is an epoxy phenolic resin prepared in the manner of Example XXIX in US. application Ser. No. 375,191, filed June 15, 1964, now abandoned, is charged positively and exposed to a light and shadow image in conventional manner. The resultant electrostatic latent image is developed in the manner described in Example I with the following composition:

Parts by weight Light mineral oil 45 Microlith CT 27 Ganex 23 Methyl Violet Tannate 5 Paraflint RGW l Paraflint RGW is a synthetic wax manufactured by Moore and Munger.

The developer on the photoconductor is transferred to bond paper in image configuration. Thereafter the photoconductor is cleaned with a web held against the photoconductor. The web contains a layer of loosely bound diatomaceous earth. The plate is then repeatedly subjected to the charge, exposure, transfer and clean cycle for 55 cycles. The resulting prints have image density of about 0.8, background density of less than about 0.01 and a resolution of about 6 line pairs per millimeter. The xerographic plate shows no significant wear and no significant charge in print quality is observed.

A comparison of Examples I and II clearly demonstrate the effectiveness in cleaning the imaging surface and also the resultant cycling ability obtained. It may also be appreciated that the instant invention provides a very cheap, fast, efficient cleaning operation and one which does not encounter liquid handling problems. In addition, a cleaning technique which removes substantially all the residual developer without contamination or degradation of the imaging surface is provided.

Although specific materials and operational techniques are set forth in the above exemplary embodiments using the developer composition and development techniques of this invention, these are merely intended as illustrations of the present invention. There are other developer materials and techniques such as recycling the absorbent powders and those listed above which may be substituted for those in the examples with similar results.

While the foregoing examples describe the use of a patterned surface developer applicator, it is also contemplated that other techniques may be employed. For example, the entire imaging bearing surface may be contacted with an appropriate liquid developer in either the conventional electrophoretic or the selective wetting techniques described above.

Other mondifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.

What is claimed is:

1. The method of cyclically developing electrostatic latent images on a reusable electrostatographic imaging surface comprising the steps of forming an electrostatic latent image on the imaging surface, developing the image with a liquid developer, transferring the developer from the imaging surface to a receiving surface in image configuration, contacting the imaging surface with a finely divided, liquid developer absorbent silica powder, having a surface area of from about 30 to about 950 mF/gm. to absorb substantially all the residual liquid developer on the imaging surface, removing the absorbent powder containing absorbed residual liquid to prepare the imaging surface for the next imaging cycle and repeating the method sequence at least one additional time, said powder being insoluble in said liquid developer and relatively nonabrasive with respect to said imaging member.

2. The method of claim 1 wherein the powder has an average particle size of from about 3 to about 15 microns.

3. The method of claim 1 wherein the imaging member is a dielectric overcoated photoconductor.

4. The method of claim 1 wherein said imaging member is wiped with a cleaning web having said absorbent powder present on its surface.

5. The method of claim 1 wherein said absorbent powder is a silica gel.

6. The method of claim 5 wherein said silica gel has a particle size of from about 3 to about 11 microns and a surface area of from about 200 to about 350 mF/gm.

7. The method of claim 1, wherein the powder has a surface area of from about to about 800 m. /gm.

8. The method of claim 1 wherein the powder has an average particle size from about 1 to about 20 microns.

9. The method of claim 1 wherein the powder has an absorbing capacity of from about 40 to about 500 mg. liquid developer per 100 mg. powder.

10. The method of claim 1 wherein the imaging surface is a reusable photoconductor.

11. The method of claim 10 wherein the photoconductor is selected from the group consisting of selenium and selenium alloys.

References Cited UNITED STATES PATENTS 10 Ellis 1347 Insalaco 1347 Bolton et a1. 15--3 Tomanek et al 1347 Rubin 95-1.7 Clark et al. 118637 Bishop 134-7 Jons et a1. 118637 10 JOHN C. COOPER, Primary Examiner US. Cl. X.R.

96--1 R; 134--79; 35515; 11737 L Y; 252-410; 

